The present invention relates generally to AC relays and more particularly to AC electromagnetic relays of the type which are closed when deenergized and which are opened as the AC current ramps upwardly to a certain level and for example which are operated by a solar photocell to automatically switch one or more outdoor lights off and on (by opening the relay when the solar light increases to a certain intensity and reclosing the relay when the solar light decreases to a certain intensity).
Photocell operated relays are commonly used for controlling street lights. To minimize the total system cost, a separate AC relay is customarily provided for each street light. Also, cost and other considerations dictate that a relay with a normally closed relay switch be used and the relay be connected to turn the light on when closed and turn the light off when open. Such an arrangement ensures that the light will be on when needed and permits using the light, if on in the daytime, to indicate a malfunction. Typically, the photocell is connected in series with the relay so that the AC voltage across the relay is dependent on the photocell resistance and thus the intensity of the light received by the photocell. As the light intensity increases, the relay current increases or ramps upwardly. As the light intensity decreases, the relay current decreases or ramps downwardly. The relay is opened when the relay current increases to a certain level and recloses when the relay current decreases to a certain lower level.
When a conventional, normally closed AC relay is operated by an upwardly ramping AC current as described, just before the AC current reaches the required level to open the relay, the relay armature can vibrate sufficiently to cause the mating contacts of the normally closed relay switch to chatter. Such incipient armature vibration and switch chatter is caused by the varying magnetic field across the working gap of the relay and the resulting ripple actuating force. In a solar photocell controlled system, such incipient armature vibration and switch chatter can occur for a significant period of time due to the gradual increase in the solar light intensity to the level required to open the relay. Also, such switch chatter can significantly reduce the operating life of the relay and the operating life of the outdoor light or other electrical device controlled by the relay. Attempts to prevent or minimize such incipient armature vibration and switch chatter have included using nickel-iron alloys for all or part of the ferromagnetic structure of the relay to flatten the permeability curve of the ferromagnetic structure, in relationship to the relay current, at the current level where such incipient switch chatter can occur. However, nickel-iron alloy parts are expensive in relationship to conventional soft iron parts due to the higher cost of the material and substantially longer annealing period required.